In iOS 7, you can go into SETTINGS, then select MAIL, CONTACTS, CALENDAR. Go to the MAIL heading, where you can toggle off LOAD REMOTE IMAGES. You can turn on load images at the bottom of individual emails if there is something that you would like to see.

"Emails opened in Mail app for iOS default to loading all images attached to that message. This makes emails format and arrange themselves as the sender intended, often with nice little header graphics and signature files, but it has a potentially serious downside: increased bandwidth usage. On a wi-fi connection that bandwidth usage hardly matters, but on many of the smaller and more limited cellular data plans, each KB and MB of data transfer is precious, and the little cutesy images and styling that comes over with many emails does nothing but eat up a data plan. There’s a simple solution to that problem though, and that means disabling remote images from being loaded into Mail app on the iPhone and iPad. "